One of 12 books developed for use with the core material (Book O) of the Elements of Mathematics Program, this text covers material well beyond the scope of the usual secondary mathematics sequences. These materials are designed for highly motivated students with strong verbal abilities; mathematical theories and ideas are developed through problem situations and explanatory materials. In this short volume the definition of relation, domain, range, and converse relation is reviewed. Properties of equivalence relations, order relations, and functions are developed. (SD)